Besides the figures subject to calculations or measurements, there are some dimensionless coefficients resulting from practice for each family of fans. They are used to make the liason between the functional parameters (flow and pressure), geometric and kinematic parameters. For example, the differences between the installed power of a particular engine, which is the same and the different kind of power, which are different, absorbed by the shafts of the various types of industrial fans are embodied in these coefficients.